* Memory Management

Both GC need to live in harmony:

- OCaml values that are stored on the Java side must but registered on
  the OCaml side with caml_register_generational_global_root(),
  otherwise, they can be reclaimed when they become unreachable from
  OCaml. If these are still reachable from Java, the java reference is
  now dangling.

- Java Objects that are stored on the OCaml side must be registered
  with jni::env().NewGlobalRef() for the same reason (if the become
  unreachable from Java, their reference becomes dangling in the OCaml
  code).

- When Java code is done with an OCaml value,
  it should call caml_unregister_generational_global_root()

- When OCaml code is done with a Java value, it should call
  env().DeleteGlobalRef()

Registering global references is costly (in both cases) so we avoid
doing that for every node. We must do it on the Java side:
- for the automaton
- for the tree (i.e. an OCaml value holding a pointer to the root of
  the document
And on the OCaml side:

- for the pointer to the Document node (root) held in an OCaml block.
  When the latter is reclamed we delete the GlobalRef associated to
  the pointer.

- for every MutableNodeList as well as the original NodeList, since
  they hold pointers to the document. In particular, MutableNodeList
  are created purely on the OCaml side and only returned as a final
  result.


**** TODO: At the end of the evaluation of the automaton, no OCaml code
runs anymore, so the OCaml GC does not get a chance to run. One way to
ensure that OCaml's GC runs one last time, is to call Gc.{minor,
major, full_major}, either explicitely or when the automaton is
reclaimed by Java.

* Signals

The JVM uses straps signals (e.g. to raise exceptions instead of
segfaulting etc.).  However the OCaml runtime also installs a signal
handler and sometime gets signal targetted at the JVM. The solution to that
is 'Signal chaining'

http://docs.oracle.com/javase/7/docs/technotes/guides/vm/signal-chaining.html

**** TODO: link against -ljsig instead of relying on LD_PRELOAD


* Exceptions

Java exceptions are never checked for in the JNI code.

**** TODO:
We should perform:
  if  (env().ExceptionCheck()) { ... }
in the JNI code. However doing so for every call to Java might kill performances


* Makefile
The makefile is buggy and sometimes need to be run twice
**** TODO: fix

* Stack Size
The OCaml code is heavily recursive. It might cause stack overflows of the JVM stack,
and if signal are not handled properly, a segfault instead of an exception. In case of
segfault, first increase the stack size with -Xss100m or so.